An electro-photographic sheet of paper duplex printing machine for printing both sides of a sheet of paper by an electro-photographic method is heretofore disclosed in JP 2009-163064 A, JP 2013-107760 A and JP 2015-11178 A. In addition, in the following explanation, the sheet of paper will be explained simply as a sheet.
The duplex printing machine disclosed in JP 2009-163064 A and JP 2013-107760 A, comprises a sheet supply section, an electro-photographic printing section, a print sheet carrying section, a print sheet discharge section, a sheet inversing (turnover) section continued from the print sheet carrying section and performing a switchback for inversing front and back sides (up and down) of an one side printed sheet by reciprocating the sheet through a switching path, and an one side printed sheet carrying path for carrying the one side printed sheet inversed up and down to the printing section, and is configured to invert the one side printed sheet and then to carry to the printing section, then to print on its inversed side and then to carry to the print sheet discharge section as a both sides printed sheet.
The duplex printing machine disclosed in JP 2015-11178 A, comprises a sheet supply section, a sheet carrying section, an electro-photographic printing section, a sheet carrying section for inversed sheet to feed the one side printed sheet and to drop it onto the sheet carrying section, a printed sheet carrying section and a print discharge section, and is configured to inverse and carry the one side printed sheet to the printing section, then to print on its inversed side for making a both side printed sheet, then carry it to the print sheet discharge section as the both sides printed sheet through the printed sheet carrying section.
The duplex printing machine disclosed in JP 2009-163064 A and JP 2013-107760 A, the one side printed sheet is turned over by reciprocating (switching back) the sheet through a switching path and carries the one side printed sheet to the printing section, when printing the other side of the one side printed sheet.
However, in above mentioned structure in which the one side printed sheet is turned over and carried to the printing section, only one piece of the one side printed sheet at most is permitted to be in the switchback path, the following one side printed sheet cannot be sent into the switchback path until preceding one piece of the one side printed sheet passes through the switchback path.
Therefore, a carrying interval of the sheets is lengthened, as a result, the time required for print processing number of sheets per time becomes longer, resulting in a reduction of print efficiency.
The duplex printing machine disclosed in JP 2015-11178 A, the one side printed sheet is turned over by dropping the sheet during carrying it onto the sheet carrying section and carries the front and back inversed sheet to the printing section, when printing the other side of the one side printed sheet.
However, in above mentioned structure in which the one side printed sheet is turned over and then carried to the printing section, since the sheet carrying section carries alternately a prescribed number of sheets, which are sheets fed from the sheet supply section and the sheets being dropped from the sheet turnover carrying section, the sheet carrying section is needed always to continue to work for sake of efficiency of the print and so forth.
Therefore, since the one side printed sheet is dropped onto the sheet carrying section during its carrying operation when printing the other side of the one side printed sheet, register of the top and bottom of the one side printed sheet is rendered to be unstable and so the register of the top and bottom of the one side printed sheet may come out of alignment.
Besides, the one side printed sheet may be piled up on the other one side printed sheet dropped in ahead, if the space in the carrying direction between the one side printed sheets dropped onto the sheet carrying section is small, thus the space in the carrying direction between the one side printed sheets should be made wide.
In addition, the interval between the sheets of the predetermined number in the carrying direction according to a time interval from the completion of feeding of the sheets from the sheet supply section to the start of feeding of subsequent sheets should also be made wide. In other words, a time interval from the completion of feeding of a prescribed number of sheets from the sheet supply section to starting of feeding of subsequent of prescribed number of sheet from the sheet supply section should be made wide to make wide the space in the carrying direction between the prescribed number of sheets.
Therefore, the print efficiency at the time of the duplex printing is reduced.
In addition, in the duplex printing machine disclosed in JP 2015-11178 A, the printed sheet carrying section and the sheet carrying section for turnover are individually arranged and the temporary fixing devices for temporarily fixing the printed image are individually arranged at two places, therefore it has high cost and setting space becomes large.
Furthermore, the duplex printing machine disclosed in JP 2015-11178 A is configured to print the other sides of the same number of the inversed one side printed sheets after having printed one sides of the several sheets in succession and then to fix the images printed on the both side printed sheets by using the fixing device, when performing a duplex printing.
In this configuration, several both side printed sheets are successively carried to the fixing device, thereafter no sheet is carried for a certain time which is same time length as that for carrying sheets. After laps of the certain time, several both side printed sheets will be carried in succession again.
In this case, a temperature change in the fixing device at the time of printing is large, because the temperature change is repeated such that the temperature in the fixing device decreases temporarily when the both side printed sheets pass through the fixing device in succession and rises again when no both side printed sheet pass through the fixing device, and thereby difference in the quality of the product is caused.
The present invention is made to solve the problems mentioned above and seeks to provide an electro-photographic sheet of paper duplex printing machine in which a duplex printing can be efficiently performed without out of alignment of top and bottom register of the one side printed sheet at the time of a duplex printing, and its cost is small and its setting space can be small, still more, the temperature change in the fixing device at the time of printing is small and thereby the quality of the product can be stabilized.